A) Field of Invention
The present invention relates to a two phase motor, and more particularly to an excitation circuit for balancing the phase voltages in a two phase motor.
B) Description of Related Art
FIG. 1 shows a conventional two phase motor 1 and corresponding excitation circuits 3 and 5. The motor 1 contains two motor windings L1 and L2. A voltage +V and -V is supplied to each excitation circuit 3 and 5 by an external battery or power supply as is known in the art.
The excitation circuit 3 contains two capacitors C1 and C3 to create a positive and negative filtered voltage, as well as two switches S1 and S2. Similarly, the excitation circuit 5 contains two capacitors C2 and C4 to create a positive and negative filtered voltage, and two switches S3 and S4.
As is known in the art, the switches S1-S4 are typically either transistors or power MOSFET's. The switches S1-S4 can also be IGBT's.
In the conventional device shown in FIG. 1, the switches S1-S4 are controlled to create a Phase I voltage and a Phase II voltage. In particular, during operation, a Phase I voltage is generated by controlling switches S1 and S2 to alternatively connect the winding L1 to the positive and negative voltage supplies +V and -V. Correspondingly, a Phase II voltage is generated by controlling switches S3 and S4 to alternatively connect the winding L2 to the positive and negative voltage supplies +V and -V.
The switches S1-S4 are controlled in a manner to create a Phase II voltage which has a 90.degree. phase shift from the Phase I voltage. FIG. 2 displays waveforms of drive signals used to control the positioning of switches S1-S4. As is known in the art, the drive signals can be produced by a microprocessor or integrated circuit.
FIG. 2 also shows the corresponding positioning of the switches S1-S4 at time periods A-D. Table 1 below summarizes the positioning of switches S1-S4 during time periods A-D.
TABLE 1 ______________________________________ S1 S2 S3 S4 ______________________________________ A Open Closed Open Closed B Closed Open Open Closed C Closed Open Closed Open D Open Closed Closed Open ______________________________________
In the conventional device shown in FIG. 1, the inductive effects of motor windings L1 and L2 cause undesirable imbalances in the Phase I voltage. These imbalances become apparent by reviewing the operation of the excitation circuits 3 and 5 during time periods A-D. This review is presented below.
FIG. 3 shows a conventional device during time period A when switches S1 and S3 are open and switches S2 and S4 are closed. During time period A, the motor phase windings L1 and L2 are respectively connected to the negative voltage supply -V. This results in inductor currents ICA being built-up in windings L1 and L2.
Referring to FIG. 4, during the transition from time period A to time period B, switch S2 opens and switch S1 doses. As a result, motor winding L1 is disconnected from the negative voltage -V and connected to the positive voltage supply +V. When this occurs, inductor currents ICB begin to build-up in windings L1 and L2.
During this transition, the inductor current in winding L1 is required to change directions. However, it is not physically possible for the current to instantaneously change directions given the presence of a residual current from ICA. As a result, the residual current from ICA raises the voltage on C1 and causes a voltage imbalance (i.e., overshoot) to appear on the Phase I voltage. This voltage imbalance is identified on FIG. 5 as reference numeral 7.
During the transition from time period B to time period C, as shown in FIG. 6, the switch S3 closes and switch S4 opens. This results in the winding L2 being disconnected from the negative voltage supply -V and being connected to the positive voltage supply +V. When this occurs, inductor currents ICC begin to build up in windings L1 and L2. However, this does not 20 generate a voltage imbalance on the Phase II voltage since the current residual from ICB flows from winding L2 through switch S3 and S1 and into winding L1.
Referring to FIG. 7, during the transition from time period C to time period D, the switch S1 opens and the switch S2 closes. As a result, motor winding L1 is disconnected from the positive voltage supply +V and connected to the negative voltage -V. When this occurs, inductor currents ICD begin to build-up in windings L1 and L2.
During this transition, current in the winding L1 is again required to change directions. However, here again, it is not physically possible for the current to instantaneously change direction given the presence of a residual current from ICC. As a result, the residual current from ICC flows through switch S2, raises the voltage on C3, and causes a voltage imbalance (i.e., overshoot) to appear on the Phase I voltage. This voltage imbalance is shown in FIG. 5 as reference numeral 9.
The conventional devices shown in FIGS. 1-7 and described above obviously have certain drawbacks. Most notably is that the fact that energy stored in motor winding L1 gets transferred to capacitors C1 and C3 during a transition from time periods A to B and from time periods C to D. As a result, an unwanted imbalance appears on the Phase I motor voltage.
As is known in the art, the energy stored in motor winding L1, which gets transferred to supply capacitors C1 and C3 and causes the voltage imbalance, can be absorbed by increasing the size of the capacitors C1 and C3 by roughly a factor of 10. However, this known solution has a significant drawback In particular, this known solution significantly increases the size of the electronics package.
It has also been known to absorb the energy stored in the motor winding L1 by placing a zener diode or transistor clamp on the power supply rail voltages. This known solution also has drawbacks. That is, when this solution is implemented, the zener diodes or transistor clamps are required to dissipate large amounts of power. This dissipation of power creates unwanted heat which in turn significantly diminishes the life cycle of the circuit.
In view of these problems, there currently exists a need for an excitation circuit which can balance the phase voltages in a two phase motor without increasing the size of the electronics package or without requiring a large amount of power to be dissipated.